Uhhh.. help, please..

Given a circle with a radius of 3, which equation expresses π as the ratio of the circumference of a circle to its diameter?
A)
= π
B)
= π
C)
= π
D)
= π

You are standing next to a really big circular lake. you want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! you decide to walk around the perimeter of the lake and measure its circumference, and find that it's 400\pi\text{ m}400? m. what is the diameter dd of the lake?